Crystallizing glass enamel compositions can be used for a variety of applications such as, for example, decorative coatings for glassware, chinaware, and the like. They are especially useful in forming colored borders around glass sheets used for automotive windshields, sidelights and backlights. The colored borders enhance appearance as well as prevent UV degradation of underlying adhesives. The crystallizing glass enamels are reactive compositions for they contain components that react and crystallize upon firing.
In general, these enamel compositions include mainly a glass frit, a colorant, and an organic vehicle. They are applied to a substrate, for example, a sheet of glass, and subsequently fired to burn off the organic vehicle and fuse the frit thus bonding the enamel coating to the substrate. Glass sheets for automotive use are generally coated with the enamel composition and then subjected to a forming process at elevated temperatures. During this treatment the enamel melts and fuses to the glass substrate and the glass is formed into a desired final shape. Such compositions can also be applied to one layer of a multilayer laminate (such as a safety glass windshield) prior to stacking the layers together, whereby the pigment/color is in the interior of the resulting multilayer laminate.
In such case, after application of the enamel by screen printing, for example, the wet film is dried or cured at low temperatures or with UV light to remove the solvents and produce a dried green enamel where the particles are held together by higher molecular weight organic binder molecules. After producing the green enamel composition layer, it can be overprinted with a conductive silver layer, and at least three additional heating steps at higher temperature are required. The first heating burns off the higher molecular weight organic binder molecules and adheres the enamel composition to the first substrate layer. A second heating allows a second glass substrate to be mated and bent together with the first. After insertion of a vinyl sheet (e.g., polyvinylbutyral) between the first and second glass sheets, a third lower temperature heating is then needed to fuse the two glass sheets and the vinyl sheet to form a glass monolith, i.e., a laminated glass windshield pane. When the glass substrates are stacked using a conventional crystallizing enamel composition when still green, upon firing to form a mated set, the enamel on the bottom sheet can undesirably adhere to the top sheet, causing damage to either or both.